Broadband wireless access technologies, offering bit rates of megabits per second or more to residential and business subscribers, are attractive and economical alternatives to broadband wired access technologies. Several variations of OFDM (Orthogonal Frequency-Division Multiplexing) have been proposed as effective anti-multipath wireless channel techniques, this is mainly because of the favorable trade-off they offer between performance in severe multipath situation and signal processing complexity. In 3GPP (the Third Generation Partnership Project) standard, LTE (Long Term Evolution) SC-FDMA (Single Carrier Frequency Division Multiple Access) and OFDM are accepted as the uplink and downlink access technologies respectively. Frequency domain equalization is a necessary receiving technique in SC-FDMA to combat the interference caused by multi-path propagation. In multi-antenna cases, linear MMSE (Minimum Mean Square Error) antenna combining is designed in frequency domain and followed by a frequency domain equalizer in a receiver. Such a receiver is often called an IRC (Interference-Rejection-Combining) receiver.
In order to perform linear MMSE antenna combining, impairment covariance is estimated on each sub-carrier in addition to the channel estimation. The impairment covariance estimation algorithms can be summarized as non-parametric (or non-structured) and parametric (structured). The non-parametric approach is used in IRC.
The basic impairment covariance estimate can be based on channel estimations for a number of sub-carriers, typically in one resource block. In conventional impairment covariance estimation methods, multiple basic impairment covariance estimates are generally averaged in order to reduce impact of noise. Simulations show that the non-parametric covariance estimation works well if impairment covariance estimates are averaged over a small bandwidth, e.g. one or two resource blocks, in interference limited scenarios. However the performance gap with an ideal IRC becomes too large when the number of involved receiving antennas increases, for example in case of eight-antenna.